Zeolites with high silica to alumina ratios, i.e., of at least six, are desirable because of their particular catalytic selectivity and their thermal stability; the latter is a property particularly important when the zeolite is used as catalyst or in adsorption procedures wherein exposure to high temperatures would be expected.
The use of quaternary ammonium salts as templates or reaction modifiers in the preparation of synthetic crystalline aluminosilicates (zeolites), first discovered by R. M. Barrer in 1961, has led to preparation of zeolites with high silica to alumina ratios which are not found in nature. For example, U.S. Pat. No. 4,086,859 discloses preparation of a crystalline zeolite thought to have the ferrierite structure (ZSM-21) using a hydroxyethyl-trimethyl sodium aluminosilicate gel. A review provided by Barrer in Zeolites, Vol. I, p. 136 (October, 1981) shows the zeolite types which are obtained using various ammonium organic bases as cation. In addition, Breck, Zeolite Molecular Sieves, John Wiley (New York, 1974), pp. 348-378, provides a basic review of zeolites obtained using such ammonium cations in the synthesis thereof, as does a review by Lok et al. (Zeolites, 3, p. 282, (1983)).
The use of tetramethyl ammonium cations (TMA) in the synthesis of zeolites A, Y and ZSM-4 (mazzite) is known, e.g., U.S. Pat. Nos. 3,306,922; 3,642,434; 4,241,036 and 3,923,639. In all these cases, the TMA is trapped in the smaller cavities in the structures (sodalite or gmelinite cages), and must be burned out at high temperatures, often leading to lattice disruption and collapse. In most of these syntheses, the SiO.sub.2 /Al.sub.2 O.sub.3 ratio of the zeolites is less than about 6.
It is also known that even minor changes in the size or charge distribution of these large organic cations can induce the formation of different zeolite structures. U.S. Pat. No. 4,046,859 teaches that replacement of one of the methyl groups of the TMA compound with a hydroxy ethyl group causes the formation of a ferrierite-like phase (ZSM-21). Many such examples are enumerated by Barrer (Zeolites, 1981). The objective of the present invention is to develop preparation methods yielding new high silica large pore materials, where the organic templates are not locked into the small cavities in the structure, but are instead present in the large "super cages" from which they can be readily removed without disruption and degradation of the host lattice.
It is a further objective of this invention to prepare materials having the basic faujasite building block (sheets of interconnected sodalite cages) linked in different ways so as to form new materials having large pores and internal free volumes. In a discussion of possible theoretical and actual structures based on interlinked trunkated cubooctahedra (sodalite cages), Moore and Smith (Mineralogical Magazine, 33, p. 1009, (1963)) showed a known zeolite built from connected sheets of linked sodalite cages in an ABCABC stacking sequence (i.e. faujasite), together with a purely theoretical structure of ABAB stacked similar sheets (this has become known as "Breck 6" by some researchers after a similar tabulation by Breck ("Zeolite Molecular Sieves", by D. W. Breck, J. Wiley and Sons, p. 58 (1973)). The latter structure comprises a hexagonal unit cell having approximate dimensions a=17.5 .ANG. and c=28.5 .ANG.. These two forms may also be viewed as being analogous to cubic (cp) and hexagonally (hp) packed sodalite cages.
As these materials comprise the same sheet only stacked in different ways, it is clear that the cp (faujasite) and hp ("Breck 6") forms may randomly intergrow to give a mixed structural composite. Said intergrowths are now well known in mineralogy, and in zeolite mineralogy in particular, thanks to the increasing use of high resolution lattice imaging electron microscopy (Millward et al., Proc. Roy. Soc., A 399, p. 57 (1985); Rao and Thomas, Accounts of Chem. Res., 18, p. 113 (1985)).
In the high silica form, the faujasite end member of this group has been described as ECR-4, and is made in the presence of several "unbalanced" alkyl ammonium template cations. The similar high silica hp form is the subject ECR-30, made in the presence of only one "unbalanced" template--vis, methyl triethylammonium. We have further discovered that, depending upon specific compositions of template and Si/Al ratios, intergrowths and mixtures of ECR-4 and ECR-30 may be synthesized and controlled. The differences in connectivity of sodalite cages in the prior art cp (faujasite, X, Y) and new hp (ECR-30) forms are clearly shown in FIG. 1.
In addition to the prior theoretical studies of the hp form, various other faujasite modifications have been discussed in the literature. One such material is CSZ-1 (U.S. Patent 4,309,313) made in the presence of cesium cations, and having an x-ray diffraction pattern which was originally tentatively indexed on a hexagonal unit cell. However, CSZ-1 was recently shown to comprise a lightly distorted faujasite structure containing a twin plane in very thin crystals (Treacy et al., J.C.S. Chem. Comm., p. 1211 (1986)). The twin creates enough strain in the faujasite lattice to cause a rhombohedral distortion (Treacy et al, in Proc. Electron Microscopy Workshop (Hawaii), San Francisco Press (1987)). A faujasite crystal with an individual double twin plane has also been observed (Thomas et al., J.C.S. Chem. Comm., p. 1221 (1981)). Other claimed faujasite like materials are ZSM-20 (U.S. Pat. No. 3,972,983) made with tetraethylammonium cations and ZSM-3 (U.S. Pat. No. 3,415,736) made with lithium and sodium. Although having a hexagonal like unit cell similar to CSZ-1 and ECR-30, the inventor of ZSM-3 could not establish a "c" axis dimension for a hexagonal cell (Kokotailo and Ciric, Molecular Sieves Zeolites-1, A.C.S. Adv. Chem. Ser. 101, Ed. Flanigen and Sand., p. 109 (1971)), and proposed that it may be a random stacking of faujasite (ABC) and "Breck 6" (AB) i.e., a random mixture of the cp and hp forms.
Recent re-evaluation of ZSM-20 by Derouane et al (Applied Cat., 28, p. 285, (1986)) and Ernst et al (Zeolites, 7, p. 180, (1987)) describe essentially the same material as being faujasite like, and comprising spherical aggregates of twinned chunky crystals, having a unit cell that can be indexed on a hexagonal unit cell. Our own analysis of ZSM-20 shows that it is an intergrown mixture of the cp and hp structures with significant intergrown crystals of the cp faujasite.
An analysis of the available data indicates that the structures and relationships between these various preparations of cp and hp stacking and sodalite cages linked through double six rings are as follows:
______________________________________ Designation Si/Al Range Structure U.S. Pat. No. ______________________________________ X 1 to 1.5 cp 2882243 Y 1.5 to 3 cp 3130007 ECR-4 3 to 10 cp pending CSZ-1 1.5 to 3.5 distorted cp 43093313 ZSM-3 1.4 to 2.25 random mix cp + hp 3415736 ZSM-20 3.5 to .infin. random mix cp + hp 3972983 ECR-30 3 to 10 hp pending ______________________________________
Morphologically ZSM-3 and ZSM-20 are similar, in that they form crystals about 0.6.mu. diameter and 0.2.mu. thick and having a squashed octahdron shape that is almost hexagonal in outline, very similar to a twinned "platelet faujasite" (U.S. Pat No. 4175059). ECR-30 and CSZ-1 are also similar morphologically, and form thin plates up to 1.mu. diameter and less than 0.05.mu. thick, as shown in FIG. 3 (ECR-30) and Treacy et al (CSZ-1) (JCS Chem. Comm., p 1211, (1986)).
A theoretical x-ray diffraction pattern for the hp structure based on the space group P6.sub.3 /mmc is shown in Table 1, assuming lattice constants of a=17.3 .ANG. and c=28.78 .ANG., and excluding water and cations. The three strongest lines are the first three peaks, having an intensity relationship of 100&gt;002&gt;101. As the 002 of the hp structure is coincident with the 110 of the cp structure, excessive intensity in this line, reflected in a relatively high 002/100 ratio, is indicative of contributions from the cp structure. An important defining characteristic of ECR-30 is therefore that this latter peak intensity ratio is minimum, and always lower than seen in mixed cp+hp structures like ZSM-3 and ZSM-20. Comparison of the intensity relationships in FIG. 2 with those for published spectra for ZSM-20 previously mentioned, clearly confirm this observation.
TABLE 1 ______________________________________ THEORETICAL X-RAY DIFFRACTRION PATTERN FOR ECR-30 (hp STRUCTURE) FOR CuK.sub.a RADIATION 20 D hkl I/I.sub.O +cp ______________________________________ 5.89 14.98 100 100 6.14 14.39 002 43.2 * 6.65 13.29 101 30.7 10.22 8.65 110 14.6 10.98 8.079 103 22.0 11.80 7.491 200 4.2 * 11.93 7.413 112 9.8 12.20 7.249 201 1.3 13.31 6.664 202 0.5 13.64 6.486 104 2.1 14.99 5.904 203 0.8 15.64 5.662 210 8.0 * 15.94 5.556 211 4.5 16.48 5.373 105 2.7 16.81 5.269 212 1.0 17.07 5.189 204 8.8 18.18 4.876 213 2.2 18.79 4.718 302 1.5 19.41 4.568 106 0.9 19.43 4.564 205 0.9 19.94 4.450 214 0.9 20.03 4.430 303 0.2 20.52 4.325 220 5.0 21.99 4.040 206 2.2 22.00 4.037 215 1.1 22.40 3.965 107 2.3 23.74 3.745 400 1.8 23.94 3.714 401 1.0 24.54 3.625 402 0.3 24.68 3.604 207 0.6 24.72 3.598 314 2.1 25.44 3.498 108 0.8 25.51 3.489 403 0.2 25.73 3.459 306 0.2 25.90 3.437 320 0.8 26.43 3.369 315 1.4 26.64 3.343 322 0.3 26.82 3.322 118 1.9 27.25 3.269 410 1.8 27.54 3.236 323 0.9 28.09 3.174 307 0.4 28.39 3.141 316 0.2 28.76 3.101 324 1.7 28.83 3.094 413 0.2 29.39 3.027 218 0.2 29.79 2.996 500 0.2 29.96 2.980 501 0.2 30.37 2.941 209 0.3 30.45 2.933 502 0.3 30.60 2.919 308 1.4 30.99 2.883 330 1.3 31.05 2.878 0010 1.9 31.25 2.860 503 3.5 31.62 2.287 1010 0.4 32.01 2.794 326 0.3 32.19 2.788 422 0.5 32.34 2.766 504/228 1.7/1.0 ______________________________________